I can't think of anything that would trigger the disappearance of content like that, especially if it's only partial, unless you deleted a content directory from the list. You weren't installing content to the application folder?

As for brushes, do you have Photoshop or are you going to use them in something else? If you have Photoshop set it up under the Applications tab in preferences; if not pick a convenient folder, tell DIM that that is your Photoshop location in the Application tab in preferences, and the brushes will go to folderyouchose/Presets/Brushes